This invention concerns a method of transmitting information contained in bits. It particularly pertains to the transmission of information between an instrument lowered in a borehole drilled in the earth and the surface. It is especially adapted to wireline well logging which is accomplished by suspending specialized devices in a well on a long cable and measuring various properties within the well in which it is drilled and transmitting the information to the surface over the wireline.
As it is well known, wells are drilled deep into the earth in the search for the production of oil and gas. It is important to perform many different operations downhole with specialized devices lowered on a wireline. It is also important to be able to control the specialized device from the surface and also of even greater importance is to transmit information detected by the device to the surface. A typical cable used for well logging is seven No. 20 gauge insulated conductors, spiral wrapped in two layers of steel wire, and may be as long as 30,000 ft. In the existing system, more than a microfarad of capacity may exist between any two conductors or between any single conductor and the steel sheath surrounding the conductors.
If more than two or three measurements or control functions or channels are to be carried out through the seven existing conductors in the example given, some method of timesharing must be used. One common method of doing so is to digitize the signals and transmit the information one binary digit or bit at a time, one channel after another. Any number of channels can be transmitted along a signal pair of wires in this fashion limited by the frequency bandwidth of the cable and the maximum amount of time that the user considers acceptable to cycle through all channels. Special transmission of binary digital data can ordinarily be accomplished by representing a binary one as one selected DC voltage signal, for example five volts and a binary zero as another DC voltage, for example, zero voltage. Digital data transmission in this manner might be observed on an oscilloscope. Due to the length and capacity of a logging cable, however, transmission of DC voltages in this fashion is somewhat impractical. The bits of information become distorted as data rates increase until the signal cannot be decoded. This is primarily a result of the limited current capacity of the transmitter not being able to drive the wireline to the appropriate voltage quickly enough and the limited bandwidth of the cable. A common method to avoid this problem is by the use of frequency modulation. The ones and zeros of the binary data are coded as bursts of two distinct frequencies. For example, the ones might be coded for 300 Hz and the zeros for 600 Hz. These signals can be detected and separated at the receiver throughout a wide range of amplitude, and the data can then be reconstructed.